As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2011 and are well known to the person skilled in the art.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated. The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to about 0.15% Ti may include an alloy having no Ti.
There are various brazing processes in use for the industrial scale manufacturing of heat exchangers.
There is vacuum brazing which is carried out at relatively low atmosphere pressure in the order of about 1×10−5 mbar or less, and is an essentially discontinuous process and puts high demands on material cleanliness. To obtain the optimum conditions for joining to take place, aluminium alloys commonly used for vacuum brazing contain purposive additions of Mg of 1% or more. The Mg destroys the hard oxide film of the filler alloy when it evaporates from the brazing sheet during brazing, and further the evaporated Mg plays the role as getter that removes oxygen and moisture remaining in the brazing furnace. There is always more magnesium present in the furnace then necessary. The excess magnesium condenses on the cold spots in the vacuum furnace and has to be removed frequently. The capital investment for suitable equipment is relatively high.
NOCOLOK™ (registered trademark) flux brazing has been used as the principal brazing process to braze automotive heat exchangers by many heat exchanger manufacturers. Major problems that have arisen from the NOCOLOK process have been flux costs, flux handling and the damage flux causes to the furnaces. Also, in complex shaped assemblies the application of the brazing flux prior to brazing at the interior of the assemblies is often considered very difficult and problematic. Consequently, most of the heat exchanger manufacturers have been trying to reduce flux consumption.
Another brazing process is controlled atmosphere brazing (“CAB”) without using a brazing flux.
European patent document EP-1430988-A discloses that for such a process of CAB without using a brazing flux the brazing sheet product used contains Mg at least in a layer constituting the brazing sheet other than the filler alloy layer, typically the core alloy contains Mg in a range of 0.05% to 1.0 wt. %. Interposed between the core alloy and the filler alloy there is present a diffusion prevention layer such a Mg-free AA3003-series aluminium alloy.
European patent document EP-1306207-B1 discloses another fluxless brazing process in an inert gas atmosphere containing a very low oxygen content. Furthermore there is disclosed a brazing sheet product comprising of an aluminium core alloy on one or both sides clad with an Al—Si alloy brazing alloy containing 0.1-5% of Mg and 0.01-0.5% of Bi as an intermediate layer, and a thin covering material clad onto the Al—Si alloy brazing alloy. It is disclosed that during a brazing operation the brazing material in the intermediate layer is molten as the temperature is elevated during brazing, but oxidation of the surface of the brazing material does not occur because the surface is covered with the thin covering material which remains solid. When the temperature is further elevated, the portions with lower melting points, such as a segregation portion of the thin covering material close to the molten brazing material, are locally molten, and then the brazing material seeps and spreads over the surface of the thin covering material due to volumetric expansion. The surface of the brazing material then becomes an emerging face without an oxidation film, and new intensive oxidation does not proceed due to the inert gas atmosphere.
An alternative approach to EP-1306207 is disclosed in each of international application WO-2008/155067, WO-2010/000666 and WO-2010/052231, and in the preferred embodiments having the combined addition of Mg and a wetting element, preferably Bi, to the Al—Si filler alloy.
There is a need for further improved brazing processes and brazing sheet materials or extruded materials for use therein.